Dissertation committee:
Έκτορας Νισταζάκης, Καθηγητής, Τμ. Φυσικής, ΕΚΠΑ
Γεώργιος Τόμπρας, Καθηγητής, Τμ. Φυσικής, ΕΚΠΑ
Άννα Τζανακάκη, Αναπλ. Καθηγήτρια, Τμ. Φυσικής, ΕΚΠΑ
Μάρκος Αναστασόπουλος, Αναπλ. Καθηγητής, Τμ. Φυσικής, ΕΚΠΑ
Χρήστος Βόλος, Αναπλ. Καθηγητής, Τμ. Φυσικής, ΑΠΘ
Ευγενία Ροδίτη, Επικ. Καθηγήτρια, Τμ. Φυσικής, ΕΚΠΑ
Βασίλειος Χριστοφιλάκης, Επικ. Καθγητής, Τμ. Φυσικής, Πανεπιστήμιο Ιωαννίνων
Summary:
The ever-growing need for more frequent and efficient communication, while using very demanding broadband applications, make the development of new telecommunication technologies a necessity. The optical wireless communications (OWC) is a technology with rapid development over the last years, mainly because of their great potential. The recent technological advancements, concerning the LEDs and LASERs, allowed the use of the optical and infrared part of the electromagnetic spectrum for transmitting the required information. Due to the high frequency of the specific part of the spectrum, the optical wireless communications offer very high data rates and theoretically infinite bandwidth, while being a more inexpensive solution with much easier installation compared to the most commonly used technologies. Moreover, the physical properties of light mean that the optical radiation is not harmful for the human health and the OWC systems offer very high security for the transmitted information, without any licensing requirements. Their numerous advantages have established them as a fundamental part of the core network of the 5G systems, as well as an alternative technology for the Wi-Fi networks, which is constantly gaining more and more ground. On the other hand, the performance and stable operation of the OWC systems is highly dependent on the weather conditions and the general characteristics of the propagation medium. The absorption and scattering of the optical beam, while propagating through the medium, limits the range of the optical systems. Another degrading phenomenon is the scintillation effect, which corresponds to the fading effect of the radiofrequency communications and is caused by the turbulence phenomenon characterizing the fluid medium. The very temporally short pulses used by the optical communications require the almost perfect synchronization between the transmitter and the receiver. However, the desynchronizations, that may occur due to the electronic devices consisting the transceivers and the various phenomena that affect the propagating pulse, constitute the time jitter phenomenon. The latter, together with the above forementioned phenomena, are studied in the current dissertation.
In the current doctorate dissertation, the cases of both free space optics (FSO) and underwater optical wireless communication (UOWC) systems, as well as hybrid communication systems are studied. The main purpose of the dissertation is to, firstly, identify the negative phenomena that affect the performance of the optical systems and afterwards extract the appropriate mathematical equations that describe their effect. After the mathematical description of the degrading phenomena, some methods and techniques to improve the performance of the systems are proposed and simulated by the appropriate simulation programming tools. Furthermore, significant emphasis is shown to the experimental verification of the theoretical results, via an experimental wireless optical link, which operates in cooperation with the Hellenic Naval Academy.
More specifically, starting from the terrestrial free space systems, the combined effect of turbulence and time jitter is studied. The exact harmful effect and the limit set by time jitter are presented for various modulation techniques. Similar results are obtained by placing relays in the middle of the total distance of the link, in order to increase the range of the system. Due to the susceptibility of the FSO systems to adverse weather and atmospheric phenomena, another system is usually adapted, which operates either in parallel with the FSO system, or when the FSO system underperforms in regard to the smooth and reliable communication. A large part of the PhD thesis addresses the performance of these systems under various conditions of turbulence, pointing errors of the laser beam and fading of the secondary employed system. The average bit error rate (ABER) and the outage probability are calculated for various statistical distributions, which describe the phenomena of turbulence, such as Gamma, Gamma-Gamma, K, Malaga and fading of the millimeter wave (MMW) signal, such as Rayleigh, Rice, Weibull.
A major achievement of the doctorate dissertation is the introduction of the time jitter effect in the underwater wireless optical communications. Initially, it is studied in combination of the chromatic dispersion which occurs in the underwater environment, due to the variations in the seawater refractive index and can lead to desynchronizations between the transmitter and the receiver. In addition, the turbulence phenomenon is added to the analysis and with the help of the performance criterion of probability of fade, its combined effect on the system is described. One of the major problems of the underwater systems is their limited range, therefore, transponders are added to the studied system and the corresponding results are presented.
The verification of the theoretical and simulation results is achieved by collecting and processing experimental data with the help of an experimental link, which is located above the sea area of Piraeus. From the processing of the data, an empirical mathematical model is extracted, which confirms the derived theoretical models. Finally, the theoretical analysis of the degrading phenomena allowed the proposal of a technique to improve the electronic circuits used in the wireless optical communication systems. By using a bi-color LED to simulate a hypersine function, the study of a chaotic system is achieved, which can be used in telecommunication applications.
Keywords:
Free space optical wireless communications, underwater optical wireless communications, hybrid FSO/MMW systems, time jitter, chromatic dispersion, turbulence
